Before we examine the differences, let’s clarify one thing. We are not comparing Universal or Bypass electrics. This article looks at the differences between type-approved dedicated electrics, electrics that are designed specifically for your vehicle and use the manufacturer’s designated connections.
There are normally two reasons why read-only towing electrics are installed.
- The vehicle does not have read-write CAN bus components installed.
- There’s no software update available for the vehicle.
Let’s now look at the differences and which one you should choose.
Understanding the Difference Between Read-Write and Read-Only Tow Bar Electrics
When fitting a tow bar to a modern vehicle, the electrical connection is just as important as the tow bar itself. Tow bar electrics link the car’s systems with the trailer, caravan, or cycle carrier you’re towing, allowing for safe lighting, power, and in some cases, enhanced safety features.
One area that often confuses is the difference between read-write tow bar electrics and read-only tow bar electrics. Both types serve the same basic purpose—providing a connection between the car and what’s being towed—but they differ in how they interact with the vehicle’s onboard electronics.
Read-Only Tow Bar Electrics
A read-only system is the most basic form of tow bar wiring. As the name suggests, it can “read” signals from the vehicle, but it cannot send any information back into the car’s control systems.
- How it works:
The tow bar electrics are wired to take signals (such as indicators, brake lights, or side lights) from the vehicle and pass them to the trailer or caravan. - Features and limitations:
- Powers external lighting (brake, indicator, tail, fog).
- Generally cheaper and quicker to install.
- Does not inform the car’s onboard systems that a trailer is attached.
- Advanced vehicle functions like Trailer Stability Assist, parking sensor suppression, or modified ABS settings will not activate.
Read-only electrics are perfectly legal and safe, but are best suited for light towing or cycle carriers where advanced vehicle integration is not required.
Read-Write Tow Bar Electrics
A read-write system is more advanced. Instead of simply taking signals from the vehicle, it both reads signals and communicates back to the car’s electronic control unit (ECU).
- How it works:
These systems are usually connected through a dedicated wiring kit designed for the specific make and model of the vehicle. The wiring is integrated with the car’s CANbus (Controller Area Network) system. - Features and benefits:
- Powers external lighting (like read-only systems).
- Informs the car that a trailer is connected.
- Activates safety and convenience features such as:
- Trailer Stability Control
- Adjusted ABS/traction control settings
- Parking sensor deactivation when towing
- Altered automatic gearbox behaviour (on some cars)
- Dashboard warnings if trailer bulbs fail
- Provides power to caravan auxiliary systems if a 13-pin socket is installed.
Because the car “knows” it’s towing, a read-write system offers a higher level of safety and integration.
Which Should You Choose?
- Read-Only Electrics:
- Best for cycle carriers or light trailers.
- Lower cost and simpler installation.
- Suitable if you don’t need the car’s advanced towing features.
- Read-Write Electrics:
- Recommended for caravans, horseboxes, or heavy trailers.
- Ensures full use of the car’s built-in safety systems.
- Ideal for modern vehicles with sensitive electronics.
Final Thoughts
The primary difference lies in communication.
- Read-only electrics let the trailer “listen” to the car.
- Read-write electrics create a two-way dialogue between car and trailer, unlocking advanced safety and convenience features.
For occasional or light use, read-only wiring may be sufficient. But if you plan to tow regularly—or your car is equipped with trailer-specific safety systems—investing in a read-write kit is the smarter, safer choice.
Safe Towing – Team Ultimate Towbars.
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